Dehydrogenation of hydrocarbons



Patented Apr. 4, 1939 DEHYDROGENATION OF HYDROOABBONS Jacque C. Morrell and Aristid V. Grosse, Chicaxo,- 111., assignors to Universal Oil Products Com- Dilly, Chicago, 111., a corporation of Delaware No Drawing. Application May 11, 1936,

Serial No. 79,217

6 Claims.

This invention relates particularly to the dehydrogenation of parailin hydrocarbons, both gaseous and liquid, although it is applicable also to the dehydrogenation of naphthenic hydrocarbons as well as to both gaseous and liquid hydrocarbon mixtures produced in the ordinary distillation or in cracking of petroleum and its fractions. It is particularly applicable to the treatment of gasoline fractions of inferior antiknock value to increase their antiknocking properties. The dehydrogenation reactions may involve isomerization and cyclization as well as the formation of oleflns and unsaturated compounds generally.

More specifically, the invention is concerned with the application of particular types of catalysts to hydrocarbon dehydrogenation reactions generally, these catalysts, whose preparation, properties and uses will be described in detail in the succeeding specification being outstanding in the matter of accelerating selective dehydrogenation reactions to the practical exclusion of ordinary thermal conversion reactions under the preferred conditions of operation.

There is a large commercial production of gaseous paraflin hydrocarbons. They occur in very large quantities in natural gas, as well as in those gases associated with the production of crudeoil and commonly known as casinghead gases, and this supply is further augmented by the gases produced in cracking oils for the production of gasoline, although this latter type of pyrolytically produced gas contains substantial quantities of olefins as well as parafiinic hydrocarbons.

The greater part of the paraiiin gas production is used merely for domestic and industrial fuel purposes and not as a source of hydrocarbon de- 'rivatives, on account of the unreactive character of its components in comparison with their 'olefinic counterparts.

The normally liquid parafiin hydrocarbons which comprise those from the pentanes of 5 carbon atoms and higher are also available in considerable quantities as substantially pure compounds from the close fractionation of highly paraflinic crude petroleum fractions and are further available in mixtures representing prodnets of less accurate fractionation. The lower boiling paraflin hydrocarbons, for example, those present in straight run gasoline and in natural gasoline are particularly amenable to treatment by the process. The present process is applicable to the production of the corresponding mono oleflns and other compounds from any of the normally liquid paraffin hydrocarbons under conditions of operation which obviously require some modification when dealing with the different compounds of this series. Cyclic compounds either of a completely saturated character such as the naphthenes or of a hydroaromatic character such as the terpenes may also be selectively dehydrogenated in whole or in part by the use of the present types of catalysts under suitably chosen conditions of temperature, pressure and time of contact.

It is more or less obvious from the foregoing statements as to the classes of hydrocarbons which can be dehydrogenated that the process is distinctly applicable to the dehydrogenation of members of the same series of hydrocarbons which occur in various percentage admixtures in petroleum fractions. Thus the catalysts to be presently described are useful in the reformingof relatively low boiling naphtha fractions either from straight run distillation or cracking processes to increase the unsaturation thereof and render them less knocking in character.

In one specific embodiment the invention comprises the dehydrogenation of hydrocarbons at elevated temperatures in the presence of catalysts comprising essentially relatively inert solid granular carriers supporting minor amounts of compounds of praseodymium.

According to the present invention, the catalysts which are preferred for selectively dehydrogenating hydrocarbons have been evolved as the result of'a large amount of investigation with catalysts having a dehydrogenating action upon various types of hydrocarbons such as are encountered either in the gaseous or liquid fractions produced in the distillation and/or pyrolysis of petroleum and other naturally occurring hydrocarbon oil mixtures. The criterion of an acceptable dehydrogenating catalyst is that it shall split off hydrogen without inducing either scission of the bonds between carbon atoms or carbon separation. In the present invention catalyst mixtures comprising essentially major amounts of inert carriers and minor amount of compounds of praseodymium, such as for example the oxides Pr04, PrzOs, PrzOs, P102, and particularly the sesquioxide PrzOa, which results from the reduction of the higher oxides. The oxides mentioned are particularly efl'icient as catalysts for the present types of reactions but the invention is not limited to their use but may employ other compounds of praseodymium which may be either deposited upon the carriers from aqueous or other solutions in the course of the preparation of the composites or which may be mechanically admixed therewith either in the wet or the dry condition. As a rule the most convenient and generally applicable method for depositing compounds of praesodymium on carriers consists in utilizing primarily aqueous solutions of such soluble compounds as the crystalline sulphates or the double alkali metal nitrates and then precipitating hydroxides, which are then ignited to form first the higher oxides and then reduced by hydrogen or the gases and vapors in contact with the catalyst in the normal operation of the process. The ignition of precipitated hydroxides, or the nitrate or oxalate, produces the black peroxide PrOz, which is readily reduced to the greenish-yellow sesquioxide PrzO: by hydrogen. Other catalytic promoting catalysts are producible by the direct ignition of the sulphates or the mixed sulphates with the alkali metals. The carbonate may be mixed mechanically and also ignited to form an oxide which is reducible to the catalytic sesquioxide. It is tobe understood that the various compounds which may be used will not have equivalent catalytic efiectlveness and that some will have greater catalytic efliciency and be more practical to use than others.

Owing to the association in natural minerals of the elements praseodymium and neodymium and the practical difiiculties encountered in the separation of these elements or their compounds, it is recognized as not likely that the compounds of praseodymium which are available for catalytic purposes will be entirely free from neodymium, but owing to the generally similar propererties of these elements the presence of the neodymium will have no deleterious influence in regard to the catalytic effectiveness of the present type of preferred composites, and apparently has some beneficial effects in some instances.

The invention contemplates the use of a number of different types of carriers for supporting the praseodymium compounds which are in effeet the active catalysts for promoting the more or less selective dehydrogenation reactions. These carriers are preferably of a rugged and refractory character capable of withstanding the severe use to which the catalysts are put in regard to temperature during service and in regeneration by means of air or other oxidizing gas mixtures after they have become fouled with carbonaceous deposits. As examples of materials which may be employed in granular form as supports for the preferred catalytic substances may be mentioned the following Magnesium oxide Aluminum oxide Bauxite Bentonite clays Montmorillonite clays Kieselguhr Crushed silica Crushed firebrick Glauconite (greensand) cining of the precipitated hydroxide or the minerals bauxite or diaspore, which are naturally occurring hydrated oxides. The clays mentioned may also require a careful calcination to preserve their structure as far as possible. In the case of silica and firebrick, these may be considered as igneous products and less sensitive to temperature effects, while glauconite should generally be ignited at a minimum temperature at least below the highest temperature of service before the praseodymium compounds are added. By using these different types of carriers alternately with the different praseodymium compounds already mentioned, the preparation of a considerable number of catalysts is made possible, although obviously they will not exert exactly equivalent effects.

Our investigations have also definitely demon strated that the catalytic eiilciency of such substances as alumina, magnesium oxide, and clays which may have some catalytic potency in themselves is greatly improved by the presence of compounds of praseodymium in relatively minor amounts, usually of the order of less than 10% by weight of the carrier. It is most common practice to utilize catalysts comprising 2% to 5% by weight of praseodymium compounds.

In making up catalyst composites of the character and composition which according to the present invention have been found specially well suited for catalyzing hydrocarbon dehydrogenation reactions, the following is the simplest and generally the preferred procedure. A properly prepared carried is ground and sized to produce granules of relatively small mesh of the approximate order of from 4 to 20 and these are caused to absorb compounds which will ultimately yield compounds of praseodymium on heating to a a proper temperature by stirring them with warm aqueous solutions of soluble praseodymium compounds, such as for example praseodymium rubidium nitrate having the formula which is sufliciently soluble in warm water to render it readily utilizable as an ultimate source of praseodymium oxides. Other soluble compounds which may be used to form catalytic deposits containing praseodymium are the various alkali metal double compounds already mentioned. Other compounds of acids of praseodymium, including compounds of the alkaline earth and heavy metals may be distributed upon the carriers by mechanical mixing either in the wet or the dry condition. While substantially all of the compounds of praseodymium will have an appreciable catalytic action in furthering dehydrogenation reactions, some will be considerably better than others in this respect and it is not intended to infer that the compounds which may be employed alternately are in any sense exact equivalents. As a rule the lower oxides are the best catalysts.

Practically all of the carriers mentioned will.

have sufliciently high absorptive capacity for solutions of soluble praseodymium compounds to take up the necessary quantities without leaving any excess of solution. Some of the salts of praseodymium which are utilizable as a source of The oxide resulting from the decomposition of such compounds as the nitrate and the hydroxide is for the most part the dioxide PrOa. This oxide, however, is reduced by hydrogen, or by the gases and vaporous products resulting from the decomposition of the hydrocarbons treated in the first stages of the dehydrogenation reactions so that the essential catalyst for the larger portion of the period of service is the sesquioxide PrzOa.

In practicing the dehydrogenation of hydrocarbons according to the present process, a solid composite catalyst prepared according to the foregoing briefly outlined methods is used as a filler in a reaction tube or chamber in the form of particles of graded size or small pellets, and the hydrocarbon gas or vapor to be dehydrogenated is passed through a stationary mass of catalyst particles after being heated to the proper temperature, usually within the range of from about 800-1200 F., depending upon the hydrocarbon or mixtures of hydrocarbons undergoing treatment. The most commonly used temperatures, however, are around 900-1000 F. The catalyst tube is usually heated exteriorly to maintain the proper reaction temperature. The pressure employed may be subatmospheric, atmospheric or slightly superatmospheric of the order of from 50 to 100 pounds per square inch. While pressures up to 500 pounds per square inch may be employed in some cases, pressures of the order of atmospheric and below are generally preferred. The time during which the hydrocarbons are exposed to dehydrogenating conditions in the presence of the preferred catalysts is comparatively short, usually below twenty seconds, and preferably from 0.5 to six seconds.

It is an important feature of the present process that the vapors of hydrocarbons to be dehydrogenated should be free from all but traces of water vapor since the presence of any substantial amounts of steam reduces the catalytic effectiveness of the composite catalysts to a marked degree; In view of the empirical state of the catalytic art, it is not intended to submit a complete explanation of the reasons for the deleterious influence of water vapor in the present type of catalyzed reactions, but it may be suggested that the'action of the steam may be to cause a partial hydration of such carriers as alumina and some of the praseodymium compounds due to preferential adsorption, so that in effect the hydrocarbon vapors are prevented from reaching or being adsorbed by the catalytically active surface.

When the process is used to selectively dehydrogenate parafilnic hydrocarbons which are normally gaseous, the exit gases from the catalytic.

tube or chamber may be passed through selective adsorbents to combine with or absorb the olefin "petroleum refining, the products may be subjected to any suitable type of fractionation to produce a final product of desired characteristics. Obviously hydrogen released by the reactions along with other light fixed gases in minor amounts may be led off from distillate receivers and the liquid products may be given any type of fractionation adequate to separate individual compounds or fractions of limited boiling range, As in the case of the olefins produced from nor-.

mally gaseous parafllnic hydrocarbons, the products of the dehydrogenation of liquid hydrocarbons may be directly contacted with any type of chemical to produce derivatives if desired.

The present types of catalysts are selective in removing two hydrogen atoms from paraffin molecules to produce the corresponding olefins without furthering to any great degree undesirable side reactions, and because of this show an unusually selective conversion of parafiins into the corresponding mono olefins as will be shown in later examples. This selectivity is particularly in evidence when dealing with normally liquid parafiin hydrocarbons such as, for example, pentanes, hexanes, heptanes, octanes, etc. The

\present preferred types of catalysts also accelerate and direct the course of the reactions leading to simple loss of hydrogen from cyclic hydrocarbon molecules so that, for example, hexahydrobenzol may be converted substantially completely into benzol without the formation of any considerable amounts of by-products or the disruption of the ring.

The procedure when employing catalysts to dehydrogenate petroleum fractionsv such as low antiknock value gasoline is generally similar to that used when treating any normally liquid hydrocarbon and consists in first vaporizing the fraction, preheating it to a suitable temperature and then passing it through a stationary bed of catalyst of sufficient extent to cause the desired development of-unsaturation and corresponding increase in antiknook value.

When the activity of the catalysts begins to diminish it is readily regenerated by the simple expedient of oxidizing with air or other oxidizing gas at a moderately elevated temperature, usually within the range employed in the dehydrogenating reactions. This oxidation effectively removes traces of carbon deposits which contaminate the surface of the particles and decrease their efliciency. It is characteristic of the present types of catalysts that they may be repeatedly regenerated without substantial loss of catalytic efficiency.

During oxidation with air or other oxidizing gas mixture in regenerating partly spent catalytic material there is evidence to indicate that the sesquioxide is to a large extent, if not completely, oxidized to the dioxide, which may combine with such basic carriers as alumina or mag-' nesium oxide to form a certain amount of various compounds of acids of praseodymium. The existence of several alkaline earth praseodymium compounds is known, but analyses have indicated that their composition is rather indefinite so that they may possibly be solid solutions or isomor-' phous oxide mixtures rather than definite chemical compounds. These compounds are later decomposed by contact with reducing gases in the first stages of service to reform the lower oxide and regenerate the real catalyst and hence the catalytic activity.

The following examples are introduced to indicate in a general way the results obtainable in practice by the use of the invention although it is not intended to limit its scope in exact correspondence with the figures presented.

l'rample I A catalyst was prepared for use indehydrogenating butane as representing normally paraflinic gases. The general procedure was to dissolve praseodymium nickel nitrate in water and utilize this solution as a means of adding a mixture of praseodymium and nickel oxides to a car rier. 9 parts by weightof the double salt was dissolved in about 100 parts of water and the solution was then added'to about 250 parts by weight of activated alumina which had been produced by calcining bauxite at a temperature of about 1300 F. followed by grinding and sizing to produce particles of approximately 8-12 mesh. Using the proportions stated the alumina exactly absorbed the solution and the particles were first dried at 212 F. for about 2 hours and the temperature was then raised to 650 F. in a period of 8 hours. After this calcining treatment the particles were placed in a reaction chamber and the mixture of oxides reduced in a current of hydrogen at about 930 F., when they were then ready for service. The dioxide of praseodymium was reduced by this treatment to the greenish yellow sesquioxide PRzOa and the nickel was reduced to the suboxide N120.

In the present instance n-butane was passed through the catalyst chamber at a temperature of 1120 F. and at atmospheric pressure so that its total contact time was about 4 secs. The composition of the exit gases as shown below indicates the selectivity of the catalytic action.

Composition of exit cases The selectivity of the catalytic action is shown by the approximately equal percentages of butenes and hydrogen and the relatively low percentages of methane, ethane and propane.

Example I! The catalyst used in this case was made by a process generally similar to that given in Example I, using, however, a burned magnesite consisting of over 90% magnesium oxide in place of the alumina, and a saturated solution of praseodymium sulfate as the means of adding a material boiling within the range of the boiling points of 5 carbon atom straight chain hydrocarbons.

aisaooa This exainple is given to show the value of present typa of catalysts in reforming low antiknock value gasolines. The catalyst was prepared by uniformly incorporating praseodymium dioxide with a bentonite clay adding enough water to moisten the mix which was then dried to evaporate excess water and heated to a temperature not exceeding 392 F. for several hours.

The dried material was ground and sized to produce approximately 10 mesh particles which were used as filler in a vertical reaction chamber.

The vapors of a Pennsylvania naphtha fraction having an initial boiling point of 180 F. and a final boiling point of 410 F. were passed downwardly through the catalyst mass at a temperature of 932 F. under atmospheric pressure and a contact time of seven seconds. The recovered naphtha fraction, which constituted 92% of the starting material, had an octane number of 71 as compared with an original value of 42 onjthefluntreated naphtha. The loss was entirely due to gas formation, the gas having an average molecular weight of 10 which shows the presence of relatively large percentages of hydrogen. The activity of the catalyst remained substantially constant for a period of 6 days after which it was restored to practically its original value by oxidizing with air at the temperature of'treatm'ent, after observing the precaution of si-eaiirriing out the tower prior to the admission 0 a We claim as our invention:

1. A process for converting normally gaseous paraflins into their corresponding oleflns which comprises subjecting the paraflins to the action of an oxide of praseodymium under conditions such as to split hydrogen therefrom.

2. A process for increasing the anti-knock value of gasoline fractions which comprises subjecting the same to the action of an oxide of praseodymium under conditions such as to lower the hydrogen content thereof.

3.. In the dehydrogenation of paraflinic hydrocarbons by splitting of hydrogen therefrom, the improvement which comprises effecting said dehydrogenation in the presence of an oxide of praseodymium.

4. In the dehydrogenation of parafllnic hydrocarbons by splitting of hydrogen therefrom, the

material containing a relatively small amount of an oxide'oi praseodymium.

5. In the dehydrogenation of paraffinic hydrocarbons by splitting of hydrogen therefrom, the improvement which comprises eifecting said dehydrogenation in the presence of aluminum oxide supporting a relatively small amount of an oxide of praseodymium.

6. In the dehydrogenation of paraflinic hydrocarbons by splitting of hydrogen therefrom, the improvement which comprises effecting said dehydrogenation in the presence of magnesium oxide supporting a relatively small amount of an oxide of praseodymium.

JACQUE C. MORRELL. ARISTID V. GROSSE. 

